For the purpose of enhancing color rendering properties of a white LED or enhancing color reproducibility in the case where a white LED is used as back light of liquid crystal displays, there is needed a phosphor that is excited with light corresponding to near ultraviolet to blue LEDs to emit red light, and researches on the phosphor are now being in progress. In JP-A 2009-528429 (Patent Document 1), a complex fluoride phosphor in which Mn is added to a complex fluoride represented by the formula A2MF6 (wherein A is Na, K, Rb or the like, and M is Si, Ge, Ti or the like) or the like is described to be useful.
As a method for producing the above-mentioned phosphor, Patent Document 1 discloses a method wherein a hydrofluoric acid solution including all the component elements dissolved or dispersed therein is subjected to evaporation and concentration, thereby precipitating the desired phosphor. Another production method is disclosed in U.S. Pat. No. 3,576,756 (Patent Document 2), wherein hydrofluoric acid solutions in which the respective component elements are dissolved are mixed together, and acetone as a water-soluble organic solvent is added to the mixed solution to lower solubility, thereby precipitating the desired phosphor. Further, Japanese Patent No. 4582259 (Patent Document 3) and JP-A 2012-224536 (Patent Document 4) disclose a method wherein element M and element A in the above-mentioned formula are individually dissolved in different hydrofluoric acid-including solutions, Mn is added to either of the solutions, and thereafter the two solutions are mixed together, to precipitate the desired phosphor.
In all the known processes for producing a Mn-added A2MF6 complex fluoride phosphor as mentioned above, it is necessary to use high-concentration hydrofluoric acid in a considerably large amount as compared to the amount of the phosphor obtained. The corrosive property of hydrofluoric acid and its toxicity to human bodies constitute difficulties in carrying out the phosphor production processes, particularly on a large scale.
In connection with this problem, one of the inventors relevant to the present invention found out that, when production of a red phosphor as a Mn-activated complex fluoride is carried out not essentially by a wet process but by a method of mixing raw material powders and heating the powder mixture to effect diffusion and migration of substances to thereby produce the desired complex fluoride phosphor, a major part of the phosphor production process can be carried out without using hydrofluoric acid. The finding is described in WO 2015/115189 (Patent Document 5).
On the other hand, the phosphors hitherto disclosed in the above-mentioned documents and the like, such as K2SiF6:Mn, have the following problem. The problem is that the amount of the phosphor needed for obtaining the desired red color, specifically, the amount of the red phosphor itself needed for obtaining white light as a whole in conjunction with, for example, the light of blue LED and the light of green or yellow phosphor, is large as compared to the amounts of the phosphors for other luminescent colors or as compared to the amounts of other kinds of red phosphors.
Note that related-art documents related to the present invention include the following documents.